This invention relates generally to fuses, and, more particularly, to current limiters which provide short circuit protection.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. An overcurrent condition may be attributable to an overload current, that is, an excessive current relative to a normal operating currents found in the electrical circuit and confined to the conductive paths of the electrical circuit. Additionally, an overcurrent condition may constitute a short circuit condition, or current flowing outside the normal conducting paths of the electrical circuit. Short circuit currents are also sometimes referred to as fault currents.
Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined limit, the fusible elements melt and opens one or more circuit through the fuses to prevent electrical component damage. In certain applications, fuses will not open from overload currents due to temporary surges in drawn current occurring in, for example, starting of motors and energization of transformers. The temporary surges are typically between one and six times a normal current level, and usually will not cause damage to components of the electrical circuit unless the overload condition is sustained over a length of time. Fuses typically do not react to an overload current of a short duration.
Fuses also protect against short circuit currents, which can be as high as many hundreds of times larger than normal operating currents, and therefore require rapid isolation in order to protect the electrical components.
Circuit breakers are also widely used as overcurrent protection devices to protect electrical circuits. While circuit breakers provide overload protection, known mechanical circuit breakers provide limited short circuit interruption capability compared to fuses. Adding additional fuse protection to achieve or extend a short circuit interruption rating in a circuit which is already overload protected using circuit breakers, however, increases costs of circuit protection and is typically not a space efficient solution.
An alternative solution to providing extended short circuit protection capability in a circuit protected by circuit breaker is to use higher priced circuit breakers for the application that is being protected. For most customers, the prospect of having to purchase a higher priced circuit breaker is an unattractive solution.
In one aspect of the invention, a short circuit current limiter is provided which comprises at least one fuse element which includes a first end and a second end. The fuse element incorporates one of a folded back configuration or a switch back configuration. The current limiter further comprises a plurality of contact terminals, one contact terminal electrically connected to each end of each fuse element, a terminal base for mechanically mounting the fuse elements and contact terminals, and a cover configured to engage the terminal base to protect the fuse elements.
More specifically, in one embodiment, the short circuit current limiter includes three poles and respective visual indicators to provide a status of corresponding fuse elements. Insulating barriers fabricated from a fiberglass composite material, both line-to-load and phase-to-phase, to protect the fuse elements from shorting. Alternatively, phase-to-phase barriers are fabricated in the cover. The folded back and switch back configurations of the short circuit current limiter described herein provide a size advantage over known current limiters which employ conventional straight fuse element configurations.
The short circuit current limiter provides protection from short circuits and therefore allows use of less expensive circuit breakers to provide overload protection in electrical circuit configurations. A compact and cost effective current limiter package is provided for combination with smaller sized, less expensive circuit breakers to provide an adequate, affordable, and space-saving circuit protection for both short circuit protection and overload protection. The short circuit current limiter is also configured to be compatible with International Electrotechnical Commission (IEC) style motor starter and motor contactor installations.